1. Technical Field
The present invention relates to a semiconductor device having a recess or protrusion (hereinafter may be referred to as recess/protrusion) provided on a substrate and, more particularly, to a semiconductor light emitting element that improves its external quantum efficiency by having the recess or protrusion provided on the substrate, and a substrate used therein and a method of manufacturing them.
2. Background Art
A semiconductor element, i.e., a light emitting diode (LED) basically comprises an n-type semiconductor layer, an active layer and a p-type semiconductor layer stacked on a substrate in this order, and electrodes are formed thereon. Light from the active layer is discharged outside of the element from such as outwardly exposed surfaces (top surface, side surface) of the semiconductor structure and exposed surfaces (back surface, side surface) of the substrate.
In detail, when light generated in the semiconductor layer enters an interface with an angle of incidence not smaller than the critical angle with respect to the interfaces with the electrodes or the interface with the substrate, the light laterally propagates while repeating total reflection within the semiconductor layer.A part of light is absorbed during the propagation, resulting in lower external quantum efficiency.
A method of roughing the top surface and side surfaces of the light emitting diode chip has been proposed, but it may cause damage on the semiconductor layer and results in cracks and/or other trouble. This leads to partial breakage of p-n junction and reduction in internal quantum efficiency.
Accordingly, provision of recess/protrusion on a surface of a substrate of a semiconductor growing side has been considered.
Methods for providing the recess/protrusion on a growth surface of a substrate have been described in literatures below. With reference to Patent Literature 3 are: Japanese Laid Open Patent Publication Nos. 2006-066442, 2005-064492, 2005-101230, 2005-136106, and 2005-314121; and with reference to Patent Literatures 4 and 5 are: Japanese Laid Open Patent Publication Nos. 2000-331937, 2002-280609, and 2002-289540.    Patent Literature 1: Japanese Laid Open Patent Publication No. 2003-318441,    Patent Literature 2: Japanese Laid Open Patent Publication No. 2005-101566    Patent Literature 3: Japanese Laid Open Patent Publication No. 2005-047718    Patent Literature 4: Japanese Laid Open Patent Publication No.    Patent Literature 5: Japanese Laid Open Patent Publication No. 2002-280611    Patent Literature 6: Japanese Laid Open Patent Publication No. 2001-053012
Each of the patent literatures 1-6 discloses provision of the recess/protrusion on the growth surface of the substrate. Patent Literature 3 and the above-described reference literatures describe provision of an etch pit in the sapphire substrate by wet etching using phosphoric acid.
When an optical structure having light extracting feature is provided on the semiconductor structure side of the substrate surface the crystallinity of the semiconductor grown thereon may be degraded due to the recess/protrusion configuration, because the surface also functions as a crystal growth surface. In addition, if a void is present in the recess provided in the substrate, light extraction efficiency may be decreased. Also, even if the substrate surface has a structure capable of improving the light extraction efficiency, the directional characteristics of emission may be adversely affected.